1. Field of the Invention
The present invention relates to a voltage regulator including a soft-start circuit.
2. Description of the Related Art
A description is given of a related-art voltage regulator including a soft-start circuit. FIG. 3 is a circuit diagram for illustrating a voltage regulator 300 according to the related art.
The voltage regulator 300 includes a reference voltage circuit 301, a soft-start circuit 302, an error amplifier circuit 303, a voltage divider circuit 304, an output transistor 305, a ground terminal 306, a power supply terminal 307, and an output terminal 308.
The reference voltage circuit 301 includes a constant current circuit 31, NMOS transistors 32 and 33, and a resistor 34, and is configured to operate as described below to output a reference voltage VREF.
The NMOS transistor 32 is in an OFF state immediately after activation. Therefore, a gate voltage of the NMOS transistor 33 rises due to the constant current circuit 31, and the NMOS transistor 33 is placed in an ON state. When a current flows through the NMOS transistor 33, a voltage is generated at the resistor 34, and a gate voltage of the NMOS transistor 32 is controlled. Through feedback from a source terminal of the NMOS transistor 33 to a gate terminal of the NMOS transistor 32, a drain current of the NMOS transistor 33 is adjusted such that a current caused to flow by the constant current circuit 31 and the drain current of the NMOS transistor 32 are equal, to thereby generate VREF by the resistor 34.
The soft-start circuit 302 includes an analog switch transistor 35, constant current circuits 36 and 37, and a capacitor 38, and is configured to operate as described below to output a reference voltage VREF_SS for soft start.
The constant current circuit 36 charges the capacitor 38. Then, the voltage of the capacitor 38 rises linearly, and a gate voltage of the switch transistor 35 is controlled. When the constant current circuit 37 causes the current to flow continuously, the switch transistor 35 operates as a source follower circuit. Therefore, the reference voltage VREF, which is output from the reference voltage circuit 301, is output from the switch transistor 35 as a reference voltage VREF_SS for soft start, which rises gradually from the start of activation.
An error amplifier circuit 303 is configured to compare VREF_SS output from the soft-start circuit 302 and a divided voltage of the voltage divider circuit 304, and to control a gate voltage of the output transistor 305 such that VREF_SS and the divided voltage has the same voltage.
In this way, the soft start of the voltage regulator is performed (see, for example, Japanese Patent Application Laid-open No. 2011-152023 and Japanese Patent Application Laid-open No. 2005-327027).
However, in the related-art voltage regulator 300, when a current with a current value Is caused to flow through the analog switch transistor 35 by the constant current circuit 37, an ON resistance (the resistance value is denoted by Ron) is generated at the switch transistor 35, and the reference voltage VREF_SS drops from the reference voltage VREF by a value obtained by Is×Ron. More specifically, there is a problem in that, when the switch transistor 35 configured to control the soft start time is used between the output of the reference voltage circuit 301 and the error amplifier circuit 303, a difference is generated between the voltage VREF_SS of an inverting input terminal of the error amplifier circuit 303 and the reference voltage VREF due to the ON resistance of the switch transistor 35.
When a W length of the switch transistor 35 is increased in order to reduce the amount of voltage drop by the switch transistor 35, the ON resistance is reduced. However, a capacitance between the drain and the gate is enlarged, and when the capacitor 38 is externally connected, malfunction due to external noise is prone to occur. Further, when the current value Is of the constant current circuit 37 is reduced, there arises a problem in that a node between the error amplifier circuit 303 and the switch transistor 35 becomes Hi-Z to increase the possibility of malfunction due to external noise.